1. Field of the Invention
The present invention relates to an endoscope objective optical system in which the focal length is variable by moving a portion of a lens group. Due to such an endoscope objective optical system, normal viewing at a wide viewing angle and magnified viewing at a high magnification are possible.
2. Description of the Prior Art
As an endoscope objective optical system which enables normal viewing and magnified viewing, an endoscope viewing optical system, in which the focal length is varied by moving a portion of a lens group, has been known in the art. In the case where only one movable lens group is provided, the magnification range of the movable lens group generally has a magnification 1:1 in order to maintain compactness of the entire optical system, and to increase the magnification ratio while the overall length of the optical system remains unchanged. The distance between an object and the image plane (i.e., the object-image distance) becomes the shortest when the magnification of the movable lens group is 1:1. However, if the movable lens group is moved, from this shortest-distance position, in a direction along which the magnification becomes larger, the object distance becomes longer, so that the endoscope viewing optical system with the single movable lens group is unsuitable for the use in the intermediate focal length range.
In U.S. Pat. No. 6,433,937, magnification is changed by moving only one lens group, and an embodiment in which the movable lens group does not have the magnification of 1:1 is disclosed. However, in this embodiment, magnification is made larger under the condition that the object distance is made extremely shorter to the extent of about 0.8 mm (in the case where the focal length at the short focal length extremity is 1 mm). As a result, there are areas where the amount of lighting is insufficient. Furthermore, in order to achieve a wider angle-of-view at a normal viewing state, the diameter of the first lens group is increased. In Japanese Patent No. 2876252, since magnification is changed by a lens group with strong negative power, a large number of lens elements are required to correct aberrations.
In Japanese Unexamined Patent Publication No. Hei-11-295596, a lens group and an imaging device are movable; however, an imaging device, such as a CCD, requires a signal-processing circuit board, cables and so forth attached thereto, so that a strong driving force is necessary to move these elements. In particular, in the case where the lens group is driven via an actuator or motor, etc., it is important to reduce the load on the movable lens group (body). Accordingly, it is desirable for the image plane to remain at a fixed position regardless of the change in magnification.
JUPP No.2001-166203 and U.S. Pat. No. 6,353,504 are examples of the prior art in which magnification is changed by moving a plurality of lens groups.
JUPP No.2001-166203 discloses a three-lens-group arrangement of the negative-positive-positive lens groups in which the second and third lens groups are moved to change magnification; however, the optical power (hereinafter, power) of the second lens group is weak, so that the magnification ratio is small. Furthermore, field curvature is large at a magnified viewing state.
UPP No. 2001-91832 (which is a family member of U.S. Pat. No. 6,353,504) discloses a four-lens-group arrangement of the negative-positive-negative-positive lens groups in which the third lens group and the second or fourth lens group are moved to change magnification; however, since the power of the third lens group is strong, the number of lens elements is large to correct aberrations, and the overall length of the optical system becomes long.